1. Field of the Invention
This invention relates to machines which determine the unbalance in a workpiece by rotating the workpiece and by measuring either the forces reacted into the machine frame via force transducers (hard bearing machines) or by measuring the deflection of the workpiece supporting members (soft bearing machines). In particular the invention relates to an apparatus for driving (rotating) the workpiece in a hard bearing machine.
2. Prior Art
One of the major difficulties in the construction of such machines is the isolation of forces generated in the driving system from the unbalance forces to be measured. It will be appreciated that any force applied in the direction of measurement and repeating cyclically with the rotation of the workpiece will register as an unbalance in the workpiece. Cyclical forces occurring at frequencies which are of the same order as the running frequency although not phase related will generate signals from which the wanted signal can be isolated but only by use of sophisticated techniques.
Noise or unwanted signals occurring at frequencies much higher than the running speed are relatively easy to discrimate against electrically; however there is a limit to the amplitude of such disturbing signals which can be handled without overloading a pre-amplifier or signal conditioner which has to cope with the total signal.
The requirements for the drive of a balancing machine are therefore twofold:
Firstly: that the overall amount of disturbing signal in the direction of measurement be kept low and
Secondly: that the disburbing signals should contain only a minimal proportion of signals occurring at or around the rotational frequency.
The quality of isolation of signal forces from driving forces is therefore of paramount importance in the design of sensitive balancing machines for accurate work. However, it will be seen that conversely, if a simple driving system can be built which creates effective isolation, balancing machines for rigid rotors can be built to run at lower speeds, which has obvious advantages.